1. Field of the Invention
The present invention relates to an ytterbium-doped optical fiber for light amplification to which ytterbium is doped. More specifically, the present invention relates to an ytterbium-doped optical fiber in which deterioration in output, known as “photodarkening”, and non-linear optical effects are inhibited.
2. Description of the Related Art
A fiber for light amplification has a configuration in which a rare-earth element or the like is doped to a core and/or a clad of an optical fiber having an axisymmetric waveguide structure, and is used as a photoactive medium for fiber amps, fiber lasers or the like. In particular, a fiber laser using an Yb-doped optical fiber containing ytterbium (Yb) as a rare-earth element as an optical fiber for light amplification can obtain output light with high beam quality and high power output. Further, such a fiber laser has an oscillation wavelength of output light of about 1 μm which is substantially equal to that of Nd—YAG, one kind of conventional high output laser. For this reason, an Yb-doped optical fiber is expected to be practically applied as a laser medium for high output light sources applied to material processing such as welding, marking and cutting.
In fiber-type optical amplifiers or fiber lasers, a phenomenon called “photodarkening” is known. This is a phenomenon in which the transmission loss of optical fibers caused by excited light or signal light propagating in fibers is increased. When such transmission loss increases, the gain of the optical fiber, in which a rare-earth element is doped and which is an amplification media, deteriorates. That is, the output of fiber-type optical amplifiers or fiber lasers is deteriorated over time, thus causing problems from the viewpoint of reliability.
Generally, in a case where high output laser light is propagated to optical fibers, light of a wavelength different from the laser light is generated and amplified, whereby it is known that so-called “stimulated Raman scattering” is generated. As a result, problems, such as deterioration in the strength of the propagated laser output light and widening of the spectrum range of the laser light occur. That is, it is preferable that optical fibers are designed to prevent the appearance of non-linear optical effects, exemplified by stimulated Raman scattering.
Several methods for inhibiting photodarkening have been disclosed to date. For example, a method for inhibiting photodarkening by applying a specific manufacturing method called “direct nanoparticle deposition (DND)” has been disclosed (for example, see Non-Patent Document 1).
Further, a method for inhibiting photodarkening by adding a high concentration of aluminum to optical fibers has been disclosed (for example, see Non-Patent Document 2).
Further, a method for inhibiting photodarkening by adding a high concentration of phosphorus to optical fibers has been disclosed (for example, see Non-Patent Document 3).
Further, the following information has been disclosed in regard to the refractive index of silica glass.
It is disclosed that an increase in refractive index of the core can be inhibited by adding a combination of aluminum oxide (Al2O3) and diphosphorus pentoxide (P2O5) to a preform made of silica glass (SiO2) (for example, see Non-Patent Document 4 and 5). In particular, it is disclosed that as the concentrations (mol %) of aluminum oxide and diphosphorus pentoxide added are approximated to equal amounts, the refractive index approximates the refractive index of pure silicon dioxide.
Further, an optical fiber, in which rare-earth elements, germanium, aluminum and phosphorus are added to a core thereof, is disclosed in Patent Document 1. This Patent Document 1 discloses that the difference in refractive index between the core and the clad is decreased and recrystallization of rare-earth elements is inhibited by adding these elements to the core.